1. Field of the Invention
This invention relates to compositions of aqueous polyurethane dispersions containing polyurethanes having lateral OH-groups, optionally in admixture mixture with PUR-dispersions without any lateral OH-groups, which contain a combination of formaldehyde resins and blocked polyisocyanates as crosslinking agent and, optionally handle-improving agents, silicones and standard additives.
The present invention also relates to the use of these compositions for the production of light coatings by the heat-activated, one-coat reverse coating process, in which the PUR-composition is applied to a temporary support and dried, after which the (textile) substrate is laminated onto the dry polyurethane film at elevated temperature and pressure and the polyurethane laminate formed is subsequently crosslinked at a relatively high temperature.
Mixed crosslinking with formaldehyde resins plus blocked polyisocyanates makes it possible to obtain coatings characterized by outstanding substrate adhesion and wet strength values.
2. Description of the Prior Art
The production of artificial leather for various applications, such as outer clothing, shoe upper material or suitcase material, from polyurethane starting materials by the transfer process is already known. In general, the artificial leather is made up of two layers (or "coats"), namely the surface layer and the adhesive layer. In general, from 20 to 50 g/m.sup.2 of solids are applied per layer. The starting materials used are normally solutions and dispersions of one-component and also two-component polyurethanes. The two-component polyurethanes only receive their final plastic-like character through crosslinking with polyisocyanates.
A more recent technique for coating textile substrates, preferably in web form, is the so-called heat-activated, one-coat reverse coating process. In this process, the coating paste is knife-coated onto a temporary support, for example a release paper or a silicone matrix, the solvent (mixture) is evaporated off, the textile substrate is laminated onto the dry, heat-activated polyurethane film on a heated pressure roll and the polyurethane film subsequently crosslinked at elevated temperature. The advantages of heat-activated, one-coat reverse coating lie in the low spread possible, for example from 15 to 30 g/m.sup.2, which enables light coatings to be obtained, but especially in the possibility of using substrates which are unsuitable or less suitable for solvent coating or for direct coating either on account of their construction or on account of their dissolving properties (for example polyacrylate substrates or so-called coagulates). In particular, light or open-mesh woven and knitted fabrics may also be coated by this heat activated, one-coat reverse coating process without any danger of lamination failures.
Disadvantages of heat-activated, one-coat reverse coating process center above all around the adhesion problems experienced with articles produced by that process. The adhesion values vary and the wet adhesion values in particular are relatively poor. In addition, the use of solutions in organic solvents involves ecological problems.
The present invention describes suitable polyurethane compositions and their use for heat-activated, one-coat reverse coating without any of the disadvantages mentioned above.